Verify for yourself that the direction of the induced Bcoil shown indeed opposes the change in flux and that the current direction shown is consistent with the right hand rule. The induced EMF produces a current that opposes the change in flux, because a change in flux means a change in energy. Energy can enter or leave, but not instantaneously. As the change begins, the law says induction opposes and, thus, slows the change. In fact, if the induced EMF were in the same direction as the change in flux, there would be a positive feedback that would give us free energy from no apparent source—conservation of energy would be violated.
Motion in a magnetic field that is stationary relative to the Earth induces motional EMF electromotive force. As seen in previous Atoms, any change in magnetic flux induces an electromotive force EMF opposing that change—a process known as induction. Motion is one of the major causes of induction. In this Atom, we concentrate on motion in a magnetic field that is stationary relative to the Earth, producing what is loosely called motional EMF.
Consider the situation shown in. The rails are stationary relative to B, and are connected to a stationary resistor R the resistor could be anything from a light bulb to a voltmeter. Consider the area enclosed by the moving rod, rails and resistor. B is perpendicular to this area, and the area is increasing as the rod moves. Thus the magnetic flux enclosed by the rails, rod and resistor is increasing. The magnetic field B is into the page, perpendicular to the moving rod and rails and, hence, to the area enclosed by them.
Since the flux is increasing, the induced field is in the opposite direction, or out of the page. Right hand rule gives the current direction shown, and the polarity of the rod will drive such a current.
Entering these quantities into the expression for EMF yields:. As seen in Fig 1 b , F lux is increasing, since the area enclosed is increasing. Thus the induced field must oppose the existing one and be out of the page. The right hand rule requires that I be counterclockwise, which in turn means the top of the rod is positive, as shown.
There are many connections between the electric force and the magnetic force. That a moving magnetic field produces an electric field and conversely that a moving electric field produces a magnetic field is part of the reason electric and magnetic forces are now considered as different manifestations of the same force first noticed by Albert Einstein.
This classic unification of electric and magnetic forces into what is called the electromagnetic force is the inspiration for contemporary efforts to unify other basic forces. Explain the relationship between the motional electromotive force, eddy currents, and magnetic damping. Motors and generators are very similar.
Furthermore, motors and generators have the same construction. The motor thus acts as a generator whenever its coil rotates. This will happen whether the shaft is turned by an external input, like a belt drive, or by the action of the motor itself. That is, when a motor is doing work and its shaft is turning, an EMF is generated.
If motional EMF can cause a current loop in the conductor, we refer to that current as an eddy current. Eddy currents can produce significant drag, called magnetic damping, on the motion involved. Consider the apparatus shown in, which swings a pendulum bob between the poles of a strong magnet. If the bob is metal, there is significant drag on the bob as it enters and leaves the field, quickly damping the motion. If, however, the bob is a slotted metal plate, as shown in b , there is a much smaller effect due to the magnet.
There is no discernible effect on a bob made of an insulator. Device for Exploring Eddy Currents and Magnetic Damping : A common physics demonstration device for exploring eddy currents and magnetic damping. In both cases, it experiences a force opposing its motion. Only the right-hand side of the current loop is in the field, so that there is an unopposed force on it to the left right hand rule. When the metal plate is completely inside the field, there is no eddy current if the field is uniform, since the flux remains constant in this region.
But when the plate leaves the field on the right, flux decreases, causing an eddy current in the clockwise direction that, again, experiences a force to the left, further slowing the motion. A similar analysis of what happens when the plate swings from the right toward the left shows that its motion is also damped when entering and leaving the field. Conducting Plate Passing Between the Poles of a Magnet : A more detailed look at the conducting plate passing between the poles of a magnet.
As it enters and leaves the field, the change in flux produces an eddy current. Magnetic force on the current loop opposes the motion. There is no current and no magnetic drag when the plate is completely inside the uniform field. When a slotted metal plate enters the field, as shown in, an EMF is induced by the change in flux, but it is less effective because the slots limit the size of the current loops.
Moreover, adjacent loops have currents in opposite directions, and their effects cancel. When an insulating material is used, the eddy current is extremely small, and so magnetic damping on insulators is negligible. If eddy currents are to be avoided in conductors, then they can be slotted or constructed of thin layers of conducting material separated by insulating sheets. Eddy Currents Induced in a Slotted Metal Plate : Eddy currents induced in a slotted metal plate entering a magnetic field form small loops, and the forces on them tend to cancel, thereby making magnetic drag almost zero.
We learned the relationship between induced electromotive force EMF and magnetic flux. The number of turns of coil is included can be incorporated in the magnetic flux, so the factor is optional. In this Atom, we will learn about an alternative mathematical expression of the law. When the coils are stationary, no current is induced.
But when the small coil is moved in or out of the large coil B , the magnetic flux through the large coil changes, inducing a current which is detected by the galvanometer G. A device that can maintain a potential difference, despite the flow of current is a source of electromotive force. Electric generators convert mechanical energy to electrical energy; they induce an EMF by rotating a coil in a magnetic field. Electric generators are devices that convert mechanical energy to electrical energy.
They induce an electromotive force EMF by rotating a coil in a magnetic field. It is a device that converts mechanical energy to electrical energy. A generator forces electric charge usually carried by electrons to flow through an external electrical circuit. Possible sources of mechanical energy include: a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air, or any other source of mechanical energy.
Steam Turbine Generator : A modern steam turbine generator. Consider the setup shown in. Charges in the wires of the loop experience the magnetic force because they are moving in a magnetic field. Charges in the vertical wires experience forces parallel to the wire, causing currents. However, those in the top and bottom segments feel a force perpendicular to the wire; this force does not cause a current. We can thus find the induced EMF by considering only the side wires.
Diagram of an Electric Generator : A generator with a single rectangular coil rotated at constant angular velocity in a uniform magnetic field produces an emf that varies sinusoidally in time. Note the generator is similar to a motor, except the shaft is rotated to produce a current rather than the other way around. This expression is valid, but it does not give EMF as a function of time. Generators illustrated in this Atom look very much like the motors illustrated previously.
This is not coincidental. In fact, a motor becomes a generator when its shaft rotates. The basic principles of operation for a motor are the same as those for a generator, except that a motor converts electrical energy into mechanical energy motion.
Read our atom on electric generators first. Most electric motors use the interaction of magnetic fields and current-carrying conductors to generate force. Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives. If you were to place a moving charged particle in a magnetic field, it would experience a force called the Lorentz force:.
Right-Hand Rule : Right-hand rule showing the direction of the Lorentz force. Current in a conductor consists of moving charges. Therefore, a current-carrying coil in a magnetic field will also feel the Lorentz force. It might seem like a stupid question and I hope I am not wasting your time. The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil.
So I guess in your question, the answer would be "proportional to". And if the number of turns is one, then your answer is "equal to". The constant of proportionality is the number of turns in the coil. You will notice a negative sign in front of this constant which is a mathematical statement of Lenz's Law and also tells us the direction of the current flow.
Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Is emf equal to or proportional to the rate of change of magnetic flux? Ask Question. Stroke, heart failure, and tiredness are just some of the possible consequences for a person having sleep apnea. The concern in infants is the stopping of breath for these longer times. One type of monitor to alert parents when a child is not breathing uses electromagnetic induction.
A pickup coil located nearby has an alternating current induced in it due to the changing magnetic field of the initial wire.
If the child stops breathing, there will be a change in the induced current, and so a parent can be alerted. Calculate the magnitude of the induced emf when the magnet in Figure 1 a is thrust into the coil, given the following information: the single loop coil has a radius of 6.
Since the area of the loop is fixed, we see that. While this is an easily measured voltage, it is certainly not large enough for most practical applications. More loops in the coil, a stronger magnet, and faster movement make induction the practical source of voltages that it is.
If emf is induced in a coil, N is its number of turns. Conceptual Questions A person who works with large magnets sometimes places her head inside a strong field. She reports feeling dizzy as she quickly turns her head. How might this be associated with induction? A particle accelerator sends high-velocity charged particles down an evacuated pipe. Explain how a coil of wire wrapped around the pipe could detect the passage of individual particles.
Sketch a graph of the voltage output of the coil as a single particle passes through it. Referring to Figure 5 a , what is the direction of the current induced in coil 2: a If the current in coil 1 increases?
Figure 5. Referring to Figure 5 b , what is the direction of the current induced in the coil: a If the current in the wire increases? Referring to Figure 6, what are the directions of the currents in coils 1, 2, and 3 assume that the coils are lying in the plane of the circuit : a When the switch is first closed? Suppose a turn coil lies in the plane of the page in a uniform magnetic field that is directed into the page.
The coil originally has an area of 0. It is stretched to have no area in 0. What is the direction and magnitude of the induced emf if the uniform magnetic field has a strength of 1. Find the average emf induced in his wedding ring, given its diameter is 2. Integrated Concepts Referring to the situation in the previous problem: a What current is induced in the ring if its resistance is0. An emf is induced by rotating a turn, Find the magnetic field strength needed to induce an average emf of 10, V.
Integrated Concepts Approximately how does the emf induced in the loop in Figure 5 b depend on the distance of the center of the loop from the wire? Integrated Concepts a A lightning bolt produces a rapidly varying magnetic field. If the bolt strikes the earth vertically and acts like a current in a long straight wire, it will induce a voltage in a loop aligned like that in Figure 5 b. What voltage is induced in a 1.
The heat transferred will be 2.
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